IO Life and Immortality. 
their composition, and these elements are combined into 
complex organic compounds, which always contain a large 
percentage of water, are very unstable, and prone to spon- 
taneous decomposition. They are composed of hetero- 
geneous, but related, parts, termed organs, the objects 
possessing them being called organized bodies. Some of 
the lowest forms of animals have bodies whose substance is 
so uniform that they exhibit no definite organs, but this 
exception does not affect the general value of this distinc- 
tion. They are always more or less definite in shape, 
presenting concave and convex surfaces, and being limited 
by curved lines. When they increase in size, or grow, as 
we properly term it, it is not by the addition of particles 
from the outside, but by the reception of foreign matter 
into their interior and its consequent assimilation. Certain 
periodic changes, which follow a definite and discoverable 
order, are invariably passed through by organized bodies. 
These changes constitute what is known as life. All the 
objects, then, which fulfil these conditions are said to be 
alive, and they all appertain either to the vegetable or the 
animal kingdom. The study of living objects, no matter 
to which kingdom they belong, is therefore conveniently 
called by the general name of Biology, which means a 
discourse on life. And as all living objects may be referred 
to one or other of these kingdoms, so Biology may be 
divided into Botany, which treats of plants, and Zodlogy, 
which treats of animals. 
Now that we have divided all organized bodies into plants 
and animals, it becomes necessary to inquire into the differ- 
ences which subsist between them, and which will enable 
us to separate the kindred sciences of Botany and Zodlogy. 
Nothing was thought so easy by older observers than the 
determination of the animal or vegetable nature of any given 
organism, but, in point of fact, no hard-and-fast line can be 
drawn, in the existing state of our knowledge, between the 
animal and vegetable kingdoms, and it is sometimes difficult, 
